Anti-friction bearings such as rolling contact bearings support and locate translating and rotating machine elements with a minimum amount of friction therebetween. These machine elements, such as shafts, spindles, or slides commonly used in machine tools and precision equipment, are often configured to translate or rotate with controlled clearance and minimum backlash in the radial or axial directions, generally with controlled or predetermined clearance between the machine element and the rolling elements. Each anti-friction bearing is provided with a plurality of rolling elements, also known as anti-friction elements, such as balls, needles, or rollers. Paired opposing races bracket the rolling elements, which may be spaced apart one from the next by a cage, separator, retainer, or the like. Precision ground or machined raceway surfaces are often provided in the races to minimize rolling friction between the rolling elements and the races. Alternatively, precisely machined inserts may be installed in the races to provide a smooth, low-friction running surface for the rolling elements.
The load bearing capacity and life of anti-friction bearings is dependent on factors such as radial clearance and tolerance between the rolling elements and races, operating loads and environment, type of cage and cage materials, bearing size and type, bearing lubrication, and machine operating speeds. The coefficient of rolling friction of anti-friction bearings is substantially less than the coefficient of friction of sliding surfaces and is far more constant. However, a constant low coefficient of friction of predetermined magnitude is often required in those applications where stick-slip action impairs rolling performance. Accordingly, preloading anti-friction bearings provides this constant low coefficient of friction, and also increases rigidity, the ability to take moments and therefore the guidance accuracy of the bearing assembly, thereby enhancing bearing service life and performance characteristics, such as reduced frictional resistance and fatigue life. Conversely, preloading which causes uneven or overloading of the rolling members may result in pitting, spalling, or cracking at the bearing surface of the rolling members, resulting in premature bearing failure.
Heretofore, controlled clearance anti-friction bearings were assembled, and when required preloaded, by one of three conventional methods. However, these conventional methods of assembly and preloading were either time-consuming, labor intensive, imprecise, and/or inefficient in achieving the goal of economical production of anti-friction bearings manufactured to very accurate tolerances to provide a controlled clearance ("precision fit") having predetermined anti-friction characteristics.
According to one conventional method of assembling an anti-friction bearing, the races of the bearing are machined to very close tolerances to provide a controlled clearance ("precision fit"), at relatively high costs in terms of labor and equipment.
According to another conventional method, like-sized rolling members are graded and selected for each bearing or each group of bearings to provide the tolerances necessary for bearing friction loading.
A third conventional method of assembling and preloading an anti-friction bearing is to provide a plurality of bearing assembly adjusting screws between the outer races of the bearing which are then systematically tightened to approach a generally uniform friction loading over the diameter/length of the bearing. This third method is difficult to implement uniformly since the screws must be adjusted in an even manner to uniformly alter the resulting pressure on the rolling elements. Furthermore, the adjusting screws are subject to loosening due to machine-generated shock and vibration if not permanently restrained in the adjusted position, which thereby prevents any further adjustment.
Accordingly, it is desirable to provide a method of accurately and precisely setting a uniform, predetermined magnitude of bearing friction along the length/circumference of a linear/rotary anti-friction bearing assembly, and to provide an anti-friction bearing for which such adjustment may be efficiently and precisely made. It is also desirable to permanently restrain the bearing in the resulting adjusted position for the duration of its anticipated service life. That is, it is desirable to assure precision preloading with predetermined friction characteristics of the rolling elements within an anti-friction bearing assembly, thereby maximizing service life and ensuring uniform and predetermined load bearing capability with minimum assembly time and cost.